An unpatched flaw in an ATI driver was at the center of the mysterious Purple Pill proof-of-concept tool that exposed a way to maliciously tamper with the Vista kernel.
Purple Pill, a utility released by Alex Ionescu [yes, that Ionescu] and yanked an hour later after the kernel developer realized that the ATI driver flaw was not yet patched, provided an easy way to load unsigned drivers onto Vista - effectively defeating the new anti-rootkit/anti-DRM mechanism built into Microsoft's newest operating system.